FIG. 1 depicts a portion of the spinal structure of a patient. Two full vertebrae 100 are shown, with a third vertebra 102 cut away to show a cross-sectional view of an intervertebral disc 104. The intervertebral disc 104 includes an annulus fibrosus (concentric, interwoven collagenous fibers integrated with cartilage cells) 106 attached to the adjacent vertebrae 100, and a more central nucleus pulposus (a mass of degenerated collagen, proteoglycans, and water) 108 surrounded by the annulus fibrosus. Intervertebral disc 104 abnormalities have a high incidence in the population and may cause pain and discomfort if nerves are impinged upon or irritated. Disc abnormalities may be the result of trauma, repetitive use, metabolic disorders, and/or the aging process. Degenerative disc disease is generally used to refer to aging-related disc abnormalities, and this disease may include manifestations such as localized tears or fissures in the annulus fibrosus 106, localized disc herniations with contained or escaped extrusions, and chronic circumferential bulging discs.
Disc fissures occur rather easily after structural degeneration (a part of the aging process that may be accelerated by trauma) of fibrous components of the annulus fibrosus 106. Sneezing, bending, or just attrition can cause tears in these degenerated annulus fibers, thus creating a fissure. The fissure may be accompanied by extrusion of nucleus pulposus 108 material into or beyond the annulus fibrosus 106. The fissure itself may be the sole morphological change, above and beyond generalized degenerative changes in the connective tissue of the disc. Even if there is no visible extrusion, biochemicals within the disc may still irritate surrounding structures. Disc fissures can be debilitatingly painful. Initial treatment is symptomatic, including bed rest, pain killers and muscle relaxants. More recently, spinal fusion with cages has been performed when conservative treatment did not relieve the pain. The fissure may also be associated with a herniation of that portion of the annulus fibrosus 106.
With a contained disc herniation, there are no free nucleus pulposus 108 fragments in the spinal canal. Nevertheless, even a contained disc herniation is problematic because the outward protrusion can press on the spinal nerves or irritate other structures. In addition to nerve root compression, escaped nucleus pulposus 108 contents may chemically irritate neural structures. Current treatment methods include reduction of pressure on the annulus fibrosus 106 by removing some of the interior nucleus pulposus 108 material by percutaneous nuclectomy. However, complications include disc space infection, nerve root injury, hematoma formation, instability of the adjacent vertebrae 100, and collapse of the intervertebral disc 104 from decrease in height.
Another disc problem occurs when the intervertebral disc 104 bulges outward circumferentially in all directions and not just in one location. Over time, the disc weakens and takes on a “rolled” shape or circumferential bulge. Mechanical stiffness of the joint is reduced and the joint may become unstable. One vertebra 100 may settle on top of another. This problem continues as the body ages and contributes to shortened stature in old age. As the disc “roll” extends beyond the normal circumference, the disc height may be compromised, and foramina with nerve roots are compressed. In addition, osteophytes may form on the outer surface of the disc roll and further encroach on the spinal canal and foramina through which nerves pass. This condition is called lumbar spondylosis.
It has been thought that such circumferential bulging disc degeneration creates segmental instability which disturbs sensitive structures which in turn register pain. Traditional, conservative methods of treatment include bed rest, pain medication, physical therapy or steroid injection. Upon failure of conservative therapy, spinal pain (assumed to be due to instability) has been treated by spinal fusion, with or without instrumentation, which causes the vertebrae above and below the disc to grow solidly together and form a single, solid piece of bone. The procedure may be carried out in conjunction with a discectomy. Other treatments include discectomy alone or disc decompression with or without fusion. Nuclectomy can be performed by removing some of the nucleus pulposus 108 to reduce pressure on the annulus fibrosus 106. However, potential complications from any of these treatments may include disc space infection, nerve root injury, hematoma formation, and instability of adjacent vertebrae 100.
These interventions have been problematic in that alleviation of back pain is unpredictable even if surgery appears to have been successful. In attempts to overcome these difficulties, new fixation devices have been introduced to the market, including, but not limited to, pedicle screws and interbody fusion cages. Although pedicle screws provide a high fusion success rate, there is still no direct correlation between fusion success and patient improvement in function and pain. Studies on fusion have demonstrated success rates of between 50% and 67% for pain improvement, and a significant number of patients have increased pain postoperatively. With the increasing life expectancy of the population, such degenerative disc disease and impairment of nerve function are becoming major public health problems. Therefore, methods of helping patients with degenerative disc problems, other than those just described, should be explored.
An example of an invasive application of energy for treating degenerative disc problems is disclosed in U.S. Pat. No. 5,433,739, issued Jul. 18, 1995 to Menno E. Sluijter et al. (hereafter referenced as “the '739 patent”). The '739 patent discloses placement of a radiofrequency (RF) electrode in an interior region of the intervertebral disc approximately at the center of the disc. RF power is applied, and heat then putatively spreads out globally throughout the disc. The '739 patent teaches the use of a rigid shaft which includes a sharpened distal end that penetrates through the annulus fibrosus and into the nucleus pulposus. In one embodiment, the shaft has to be rigid enough to permit the distal end of the RF electrode to pierce the annulus fibrosus, and the ability to maneuver its distal end within the nucleus pulposus is limited. In another embodiment, a somewhat more flexible shaft is disclosed. However, neither embodiment of the devices of the '739 patent permits access to the posterior, posterior lateral and posterior medial region of the disc, nor do they provide for focal delivery of therapy to a selected local region within the disc or precise temperature control at the annulus. The '739 patent teaches the relief of pain by globally heating the disc.
An example of a noninvasive application of energy for treating degenerative disc disease is disclosed in U.S. Pat. No. 6,254,553, issued Jul. 3, 2001 to Lars Lidgren et al. (hereafter referenced as “the '553 patent”). The '553 patent discloses an ultrasound transducer which can focus an ultrasonic field in an intervertebral disc, preferably in the nucleus pulposus, for heating the tissue therein. Portions of the intervertebral disc are heated to temperatures in the range of 45-80° C. so that the tissue in the focal area degenerates, thus reducing the pressure in the intervertebral disc and, in turn, reducing pressure against the spinal cord. To provide the ultrasonic energy, the patient is placed upon a treatment table and the ultrasonic transducers, along with monitoring and positioning equipment as desired, are suspended from a frame. The frame is movable with respect to the treatment table to compensate for patient positioning and movement, as the focal area must be precisely located, to avoid unintended heat damage to the patient's tissues.